This invention relates to superconducting magnet assemblies suitable for magnetic resonance imaging (hereinafter called "MRI"), and more particularly to a cryocooler positioning mechanism for such a superconducting magnet.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other liquid cryogen. The extreme cold ensures that the magnet coils are maintained in superconducting operation, such that when a power source is initially connected to the magnet coils for a short period of time to introduce a current flow through the coils, the current will continue to flow through the coils even after power is removed due to the absence of electrical resistance in the coils, thereby maintaining a strong magnetic field. Superconducting magnet assemblies find wide application in the field of MRI.
Considerable research and development efforts have been directed at minimizing the need to replenish the boiling cryogen such as helium. This has led to the use of cryogen gas recondensing systems utilizing a mechanical refrigerator or cryocooler to cool the cryogen gas and recondense it back to liquid cryogen for reuse.
However, from time to time it becomes necessary to remove the cryocooler for replacement and/or servicing. It is desirable to accomplish this without discontinuing superconducting operation of the magnet because of the time and expense resulting from relatively long "down-time" and subsequent ramping up period of bringing the magnet back to superconducting operation.
However, it has proven difficult to insert a replacement cryocooler into the cryocooler sealed cavity of the operating superconducting magnet because of the interaction of the strong magnetic field present and the magnetic materials in the cryocooler. The attractive magnetic forces tend to pull the cryocooler cold head out of alignment, which during insertion leads to conditions of misalignment and poor thermal contact with the thermal interfaces for the superconducting magnet thermal radiation shield and recondenser. Also, the weight of the cryocooler (typically 45 to 47 pounds) makes proper positioning of the cryocooler difficult particularly in the presence of the strong magnetic forces. The magnetic forces when added to the weight of the cryocooler can also raise a possible safety problem for the field engineer. Moreover, the ride-through period during which superconducting operation of the magnet continues without cryogen recondensing is limited, and delays in securing proper alignment and proper thermal contact can lead to unplanned and undesired quenching of superconducting operation.
Thus, there is a particular need for cryocooler system which minimizes the difficulties in properly positioning the cryocooler in the sealed cavity, and obtaining during the short ride-through period good thermal contact between the cryocooler, magnet, and recondenser.